Electronic equipment housed within an enclosure is often cooled by one or more fans which are operated to draw or blow air through the enclosure past the equipment therein. In the past, the fan(s) within such enclosures were usually operated at their rated speed (RPM) at all times, the rated speed being selected to assure sufficient air flow to avoid overheating under worst-case conditions. Operating the fan(s) at their rated speed at all times is inefficient and wasteful of energy because worst-case conditions infrequently occur. Moreover, fan operation at full speed is usually accompanied by a high noise factor, which is generally undesirable.
In an effort to avoid the above-enumerated disadvantages, controllers have been developed for varying the fan speed (RPM). Present day fan controllers generally operate to vary the fan speed continuously, or between a low and a high speed, either in accordance with the temperature of the air entering the enclosure (T.sub.i), or the temperature (T.sub.e) of the air leaving the enclosure, or with the temperature difference .DELTA.T between T.sub.e and T.sub.i. Under normal operating conditions, each of these approaches accomplishes reduced fan speed under normal operating conditions, thereby lowering energy consumption and fan noise.
Variable fan-speed operation in accordance with each of these approaches, while preferable to fixed-speed fan operation, nonetheless incurs difficulties. For example, varying the fan speed in accordance with the air inlet temperature T.sub.i does not account for any diminution in the volume of air drawn through the enclosure as a result of a clogged fan filter or a blocked air inlet. In addition, regulating the fan speed in accordance with T.sub.i does not account for any increase in heat load, due to an increase in amount of electronic equipment within the enclosure or a fan failure.
By comparison, varying the fan speed in accordance with .DELTA.T or T.sub.e does account for a diminished air flow, as well as an increased heat load. However, varying the fan speed in accordance with .DELTA.T does not take into account an increase in the value of the inlet air temperature T.sub.i. As the inlet air temperature T.sub.i rises, it is possible that .DELTA.T may not increase. However as the inlet air temperature rises, the temperature of the electronics within the enclosure may likewise rise, giving rise for a need for greater cooling. However, if the fan speed is regulated in accordance with the actual value of .DELTA.T, the fan speed will not change. Under these circumstances, electronic equipment overheating may occur. The disadvantage to controlling the fan speed in accordance with the air exhaust temperature T.sub.e is that the technique provides no indication as to what conditions may have changed. For example, controlling the fan speed in accordance with T.sub.e would not indicate whether the increase in heat load was due to a blocked fan, an increase in the amount of electronics, etc.
Thus, there is a need for a technique for varying the fan speed which accounts for changes in both T.sub.i and .DELTA.T to achieve enhanced cooling and control.